1. Field of the Invention
This invention relates to a method of manufacturing an electronic device comprising a thin film transistor.
2. Description of the Prior Art
Thin film transistors (TFTs) and their methods of manufacture are well known in the field of large area electronics. Applications include the use as switching elements in active matrix devices such as displays and image sensors for example, in which TFTs are arranged in a row and column array and are addressed by selection and data signals via sets of row and column address conductors respectively. There is much interest in improving the performance characteristics of TFTs in order to produce higher quality active matrix devices which can be addressed faster, have lower power consumption and are more reliable.
The semiconductor layer of a TFT is commonly formed from hydrogenated amorphous, polycrystalline or single-crystal silicon. The hydrogenation occurs at some stage during the manufacturing process and serves to electrically neutralise the semiconductor material. This improves the TFT's characteristics including higher carrier mobility, lower threshold voltage and lower leakage current.
Hydrogenation of TFTs is commonly achieved by heating the devices in hydrogen gas to temperatures around 250° C. or higher, or by exposure to atomic hydrogen in a plasma at 300° C. or higher. However, these temperatures are too high for use with substrates having low heat resistances such as polymer substrates for example.
US 2002/0004289-A1 discloses a method of manufacturing a TFT in which a hydrogen-containing film is formed over the TFT after electrodes are formed on the source and drain regions of the semiconductor layer. The hydrogen-containing film is irradiated with a pulsed laser beam causing the hydrogen to diffuse into the semiconductor layer. This is executed at a temperature lower than the heat-resistant temperature of the substrate.
However, in the method of US 2002/0004289-A1 the hydrogen must diffuse under the metal gate and along the length of the active channel region of the semiconductor material located between the source and drain regions in the case of a top-gated device, or under a silicon oxide film in the case of a bottom-gated device. In order for this to be achieved without heating the device to an excessive temperature, a large number of low power laser pulses are required. This reduces the throughput of the manufacturing process.